Formwork for Concrete Structures by R.L.Peurifoy and G.D- By EasyEngineering.net

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Forms for Walls 261
The commercially available Jahn system of patented hardware
attachments is illustrated in Figure 9-26. The Jahn “A” bracket, patented
in Canada, is shown in Figure 9-26(a). It can be installed either
before or after walers are in place. The slots in the bracket allow it to
slip easily over the snap tie end, eliminating laborious threading
through a hole. Pressure of the bracket body is against the 2 × 4 instead
of the plywood. The bracket can be used for any type of wall forms:
round, curved, battered, beams, or columns.
Figure 9-26(b) illustrates the U.S. patented Jahn “C” bracket,
which is used to attach vertical strongbacks for formwork alignment.
It is designed for use with single 2 × 4 studs, double 2 × 4 wales, and
snap ties. The eccentric bracket securely holds formwork while compensating
for minor variations in lumber sizes. The bracket and double
wales can also be used to support a horizontal plywood joint.
Figure 9-26(c) illustrates the Jahn cornerlock for securing 2 × 4
walers. Only two nails are needed for attachment, whereas barbed
plates grip the side of the 2 × 4 lumber for positive nonslip action. The
locking handle has a cam-action, which draws the wales together at
true right angles. No special tools are needed for either installation or
stripping.
Figure 9-26(d) illustrates the Jahn footing-clip, which may be used
in place of a 2 × 4 plate for securing plywood sheathing to the top of
grade beams. Figure 9-26(e) shows the Jahn plywood-holder for
attachment of two adjacent plywood panels.
Figure 9-26(f) shows metal spreader cleats for securing ¾-in. plywood
panels for walls up to 18 in. high without snap ties. The spreader
cleats are spaced at 2-ft maximum centers.
Preparation for the forming involves gang drilling the plywood
panels with holes drilled ¹⁄8 in. larger than the snap tie head. Normally
a 5 ⁄8- in.-diameter drill bit will be adequate. The 5 ⁄8-in. take-up
on the Jahn “A” bracket allows a snap tie to be used with 5 ⁄8-in. or
¾-in. plywood. The 5 ⁄8-in. eccentric take up will also allow the Jahn
“C” bracket and snap ties to be used with 5 ⁄8-in. or ¾-in. plywood.
The most common snap tie spacings used with the Jahn forming
system are shown in Figure 9-27 for two placement rates of concrete.
The spacings shown are based on using ¾-in. Plyform Class I placed
in the strong direction, face grain parallel to the spacing, and 2 × 4 S4S
studs of Douglas Fir-Larch or Southern Pine with a minimum allowable
fiber stress of 1,200 lb per sq in. For different rates of pour, the
manufacturer should be consulted for technical assistance.
Footing plates or clips are attached to a level footing surface as a
starting point for the forms. The Jahn foot clips should be attached at
24-in. maximum spacing, with a minimum of two clips for any piece
of plywood.
The first panel that is installed in the wall form is erected in a
plumb position and braced temporarily. Each additional sheet of
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